Among the more effective antibiotics are those that interfere with common modes of bacterial gene expression, regulation or activity. Recently, the supercoiling of DNA had been suggested as a possible mode of virulence gene regulation. Local increases or decreases in DNA density, due to supercoiling, have been associated with responses to various environmental conditions such as, temperature, anaerobiosis, and osmolarity. Appropriate regulation of the accessibility of groups of genes to components of the transcriptional apparatus by increasing or decreasing supercoiling of spacially organized genes may represent an infecting pathogen's effective response to such environmental conditions. Enzymes, such as DNA topoisomerases including type 1 topoisomerases and DNA gyrases, have been identified which function to effect the levels of DNA supercoiling. Such enzymes represent useful targets against which to screen compounds as potential antibiotics.
DNA transformations performed by DNA topoisomerases are accomplished by the cleavage of either a single strand or both strands. The unit change in the Linking number (Lk) resulting from such transformations is the best operational distinction between the two classes of topoisomerases (P. O. Brown & N. R. Cozzarelli, Science 206:1081-1083 (1979)). The linking number (Lk) is the algebraic number of times one strand crosses the surface stretched over the other strand. DNA topoisomerases whose reactions proceed via a transient single-stranded break and changing the Lk in steps of one are classified as type 1, while enzymes whose reactions proceed via double-stranded breaks and changing the Lk in steps of two are classified as type 2.
Members of type 2 topoisomerase family include DNA gyrase, bacterial DNA topoisomerase IV, T-even phage DNA topoisomerases, eukaryotic DNA topoisomerase II, and thermophilic topoisomerase II from Sulfolobus acidocaldarius (see: A. Kikuchi et al., Syst. Appl. Microbiol. 7:72-78 (1986); J. Kato et al., J. Biol. Chem. 267:25676-25684 (1992); W. M. Huang in DNA Topology and Its Biological Effects (N. R. Cozzarelli and J. C. Wang, eds., Cold Spring Harbor Laboratory Press, New York, 1990), pp. 265-284; T.-S Hsieh in DNA Topology and Its Biological Effects (N. R. Cozzarelli and J. C. Wang, eds., Cold Spring Harbor Laboratory Press, New York, (1990), pp. 243-263)). The coding sequences of a dozen or so type 2 enzymes have been determined, and the data suggest that all these enzymes are evolutionary and structurally related. Topological reactions catalyzed by type 2 topoisomerases include introduction of negative supercoils into DNA (DNA gyrase), relaxation of supercoiled DNA, catenation (or decatenation) of duplex circles, knotting and unknotting of DNA.
The family of type I topoisomerases comprises bacterial topoisomerase I, E. coli topoisomerase III, S. cerevisiae topoisomerase III (R. A. Kim & J. C. Wang, J. Biol. Chem. 267:17178-17185 (1992), human topoisomerase III (Hanai et al., Proc. Natl. Acad. Sci. 93:3653-3657 (1996)), the type 1 topoisomerase from chloroplasts that closely resembles bacterial enzymes (J. Siedlecki et al., Nucleic Acids Res. 11:1523-1536 (1983), thermophilic reverse gyrases (A. Kikuchi, In DNA: "Topology and Its Biological Effects" (N. R. Cozzarelli and J. C. Wang, eds., Cold Spring Harbor Laboratory Press, New York, 1990, pp. 285-298); C. Bouthier de la Tour et al., J. Bact. 173:3921-3923 (1991), thermophilic D. amylolyticus topoisomerase III (A. I. Slesarev et al., J. Biol. Chem. 266:12321-12328 (1991), nuclear topoisomerases I and closely related enzymes from mitochondria and poxviruses (N. Osheroff, Pharmac. Ther. 41:223-241 (1989)). With respect to the mechanism of catalysis these topoisomerases can be divided into two groups. Group A consists of enzymes that require a divalent cation for activity, and form a transient covalent complex with the 5'-phosphoryl termini (prokaryotic type 1 topoisomerases, S. cerevisiae topoisomerase III, and human topoisomerase III). Group B includes type 1 topoisomerases that do not require a divalent cation for activity, and bind covalently to the 3'-phosphoryl termini (nuclear topoisomerases I, enzymes from mitochondria and poxviruses commonly called eukaryotic topoisomerases I). Type 1 topoisomerases can carry out the following topological reactions: they relax supercoiled DNA (except of reverse gyrases), catenate (or decatenate) single-stranded circular DNAs or duplexes providing that at least one of the molecules contains a nick or gap, or interact with single-stranded circles to introduce topological knots (type 1-group A topoisomerases). Reverse gyrase, belonging to type 1-group A topoisomerases, is the only topoisomerase shown to be able to introduce positive supercoils into cDNA.
Research on DNA topoisomerases has progressed from DNA enzymology to developmental therapeutics. Bacterial DNA topoisomerase II is an important therapeutic target of quinolone antibiotics; mammalian DNA topoisomerase II is the cellular target of many potent antitumor drugs (K. Drlica, Microbiol. Rev. 48:273-289 (1984) and Biochemistry 27:2253-2259 (1988); B. S. Glisson & W. E. Ross, Pharmacol. Ther. 32:89-106 (1987); A. L. Bodley & L. F. Liu, Biotechnology 6:1315-1319 (1988); L. F. Liu, Annu. Rev. Biochem. 58:351-375 (1989)). These drugs, referred to as topoisomerase II poisons, interfere with the breakage-rejoining reaction of type II topoisomerase by trapping a key covalent reaction intermediate, termed the cleavable complex. Mammalian topoisomerase I is the cellular target of the antitumor drug topotecan (U.S. Pat. No. 5,004,758), which also traps the covalent reaction intermediate.
As mentioned above, bacterial type I topoisomerases (topoisomerase I & III) are enzymes that alter DNA topology and are involved in a number of crucial cellular processes including replication, transcription and recombination (Luttinger, A., Molecular Microbiol. 15(4): 601-608 (1995). These enzymes act by transiently breaking one strand of DNA, passing a single or double strand of DNA through the break and finally resealing the break. Cleavage of the DNA substrate forms a covalent linkage between a tyrosine residue of the enzyme and the 5' end of the DNA chain at the cleavage site (Roca, J. A., TIBS 20:156-160 (1995).
Enzyme inhibition which leads to the stabilization of the covalent-enzyme-DNA complex (cleavable complex), will invoke chromosomal damage, and bacterial cell death. Furthermore, this mechanism has the potential of leading to cell death by virtue of a single inhibition event. A small molecular weight inhibitor, which acts by stabilization of the cleavable complex may act on both topoisomerase I and III because of the extensive amino acid sequence similarity between them, particularly in the region of their active sites. The likelihood of future high level resistance to such agents arising from point mutation may therefore be low.
Inhibitors of type I topoisomerases, for example, those able to stabilize the protein in a covalent complex with DNA would be lethal or inhibitory to the bacterium and thereby have utility in anti-bacterial therapy. It is particularly preferred to employ Streptococcal genes and gene products as targets for the development of antibiotics. The Streptococci make up a medically important genera of microbes. They are known to produce two types of disease, invasive and toxigenic. Invasive infections are characterized generally by abscess formation effecting both skin surfaces and deep tissues. S. pneumoniae is the second leading cause of bacteremia in cancer patients. Osteomyelitis, septic arthritis, septic thrombophlebitis and acute bacterial endocarditis are also relatively common. There are at least three clinical conditions resulting from the toxigenic properties of Streptococci. The manifestation of these diseases result from the actions of exotoxins as opposed to tissue invasion and bacteremia. These conditions include: Streptococcal food poisoning, scalded skin syndrome and toxic shock syndrome.